Sonic pressure wave generator



Jan. 25, 1966 N. HUGHES 3,230,923

SONIC PRESSURE WAVE GENERATOR Filed Nov. 21, 1962 T 1 CI. 11

INVENTOR.

Mum m 1404/ /55 ATTO EYS.

United States Patent 3,230,923 SONIC PRESSURE WAVE GENERATOR Nathaniel Hughes, Bronx, N.Y., assignor to Sonic Development Corporation of America, Yonkers, N.Y. Filed Nov. 21, 1962, Ser. No. 239,236 14 Claims. (Cl. 116-137) This invention relates to apparatus for generating pressure waves in fluids; and more particularly, to apparatus utilizing the flow of gases to generate such pressure waves in gaseous media.

Generators to which this invention relates are often called sonic or ultrasonic generators and, for convenience, they will be referred to as sonic generators throughout this description. However, it is to be understood that generators in accordance with the present invention are capable of producing pressure waves at frequencies within a wide ange which extends well beyond the normal sonic range.

A wellknown type of gas-operated sonic generator is the Hart-mann generator. (See Hartmann Acoustic Generator, Engineering 142: 491-1936; On a New Method for Generation of Sound Waves, Physical Review, S.2.V. 20, 1922, pages 719-727; and A New Acoustic Generator, J. Hartmann and Bridgit Trolle; Filed in Division 34, Class 116/ 137-A in the US. Patent Office. This Hartmann generator uses pressurized air to create a gas jet which is then directed into a cavity resonator to create a sonic output pressure wave in the surrounding air. Although this Hartmann generator has long been available, it has not been extensively used as a source of sonic energy because it is inefiicient, and because its utility is seriously restricted by the relatively high input gas pressure it requires.

Many attempts have been made to improve the Hartmann device, but they .have been relatively unsuccessful and the resulting generators have fared very little better than their predecessor.

A major objective of the present invention is to provide a truly etficient sonic generator.

A further object of this invention is to provide a gasoperated sonic generator which will operate efliciently with relatively low input gas pressures, and one which is simple and inexpensive to manufacture and use.

Other objects, aspects and advantages of the present invention will be pointed out in, or apparent from, the following description and drawings, of which:

FIGURE 1 is a perspective view of an air-operated sonic pressure wave generator embodying the present invention;

FIGURE 2 is a vertical sectional view of this generator taken along line 2-2 of FIGURE 1 in the direction of arrows; and

FIGURE 3 is a vertical sectional view of the sarne generator taken along line 3-3 of FIGURE 1 in the direction of arrows.

The air-operated sonic pressure wave generator shown in FIGURES 1 through 3 comprises a nozzle portion having a converging inlet section 12 and a diverging outlet section 14, a source of pressurized air (not shown) connected through a pipe 16 to nozzle 10, and a pulsator unit 18 attached to nozzle 10. Pulsator unit '18 includes a pulsator chamber 20 which is positioned opposite the exit of nozzle 10 to intercept the gas jet from the nozzle.

Sections 12 and 14 of nozzle 10 are each shaped like the frustrum of a cone. Inlet section 12 converges from an inlet opening 22 to an orifice 24, while outlet section 14 diverges from orifice 24 to an exit opening 26. This converging-diverging nozzle 10 is designed to convert air delivered through pipe 16 at an input air pressure P0, greater than atmospheric pressure, into an air jet having 3,230,923 Patented Jan. 25, 1966 "ice a very high speed and a pressure P at exit 26 very substantially lower than atmospheric pressure. This air jet is then directed into the pulsator chamber 20 to produce powerful sonic pressure waves in the surrounding air. This combination forms a sonic generator which ditfers from any prior generators in that, by using such a high speed jet and such a low exit pressure, it can produce pressure waves of very high intensity using much lower values of input air pressure and much lower air flow rates than those used in any prior sonic generators. The result is that generators in accordance with the present invention are substantially more efficient than any previously available. In accordance with the present invention it is believed that such high efliciency results from the provision at the exit 26 of nozzle v10 of a low output pressure. P which produces an interaction between the high-speed, low-pressure air jet and the ambient air to further increase the generators output power with a given air supply input.

Attainable air jet speeds at the exit 26 of the nozzle 10 fall within the range from somewhat greater than Mach 1 to theoretically infinitely high speeds. Jet speeds in the range of Mach 2.5 to Mach 10 have proved to be convenient for providing the high power, high efficiency output resulting from the use of the present invention. Jet exit pressures P attainable in accordance with this invention fall within the range from slightly less than onehalf of input pressure P0 to close to zero pounds per square inch absolute (p.s.i.a.). However, pressures of less than 1 p.s.i.a. have proved to be the most desirable for providing maximum power output and efficiency. It is believed that all previously-known gas-operated sonic generators produce corresponding pressures that are not only substantially higher than the very low pressures preferably used, in generators of the present invention, but also are higher than the, ambient pressure of the air surrounding them. Since this condition does not permit these prior devices to use the above-mentioned interaction between the low-pressure jet andv the higher-pressure surrounding air, these devices do not have the high efficiency and power output available in generators of the present invention.

As is well-known in the art of compressible fluid dynamies (see, -for example, A. H. Shapiro, The Dynamics and Thermodynamics of Compressible Fluid Flow, volume 1, Chapter 4; 'Ronald "Press, New York, 1953) the jet speed and pressure desired can be obtained from a converging-diverging nozzle having dimensions determined from the following equations (assuming isentropic flow in the nozzle):

A=the cross-sectional area of the nozzles conduit at any point along its longitudinal axis.

A*=the cross-sectional area of the nozzle conduit at the point where the Mach number of the gas in the nozzle=1.0.

M the Mach number of the flowing gas at any point along the nozzles longitudinal axis at which the nozzles cross-sectional area is A and the pressure of the gas flowing is P.

k=the ratio of specific heats of the gas flowing through the nozzle.

P0=the absolute pressure of the gas at the nozzle inlet (stagnation pressure).

P=the absolute pressure of the gas in the nozzle at any point along its longitudinal axis.

In order to obtain a jet having a Mach number M at the nozzle exit 26 greater than one, the gas must have a Mach number M* at the orifice 24 of at least one. A ratio of the nozzle inlet area A to the nozzle area A* at the orifice 24 (Ao/A of approximately 2/1 has proved sufiicient to create in most nozzles of a size practical for use in sonic generators a gas flow through orifice 24 having a Mach number M *=l.

The ratio of the area of exit opening 26 A to the area A* of the orifice 24 (A /A necessary to obtain the desired exit Mach number M and pressure P can be obtained from Equations 1 and 2 above. Although it is theoretically possible to use a ratio A /A* of from slightly more than one up to an extremely high value, a ratio A /A* between 3.5 and 27 has proved most desirable for use in nozzles of -a size practical for use in sonic generators. Mach numbers M and pressures P within the ranges preferred for producing powerful sonic pressure waves have been obtained with input pressures P0 lower than 1 pound per square inch gage (p.s.i.g.) and many practical sonic generators embodying the present invention use inlet pressures of less than p.s.i.g.

In determining the lengths I and L of the sections '12 and 14, respectively, of nozzle 10 necessary to obtain the preferred values of pressure and Mach number from the nozzle, care should be exercised to maintain laminar the flow in the boundary layers of gas at the interior surfaces of the nozzle. If this flow is maintained laminar, any detrimental effects of these boundary layers will be negligible. However, if this flow becomes nonlaminar or turbulent, the boundary layers grow thick and tend to occlude and flow through the nozzle and alter the nozzles flow characteristics.

The problem of boundary layer growth is not especially critical in convergent section 12 of nozzle '10 where the gas speeds are less than Mach 1. Hence, any convenient, moderate convergence angle (a) and corresponding length I (for a given area ratio Ao/A may be used. A convergence angle (a) or 30 degrees has been used successfully in nozzles of a size most practical for use in sonic generators. In the diverging secion 14 of nozzle '10, however, more care must be taken to prevent excessive boundary layer turbulence and corresponding excessive growth of the boundary layer thickness. As is well known in the art of high-speed-gas dynamics, the maximum boundary layer'thickness in the nozzle is a function of the Reynolds number of the gas flowing at the interior wall of divergent section '14. Since this maximum Reynolds number is a direct function of the length L of diverging section 14 and, for a given ratio A /A an inverse function of its divergence angle (b), it is desirable to maximize the convergence angle (a) and minimize the length L to properly control boundary layer growth. In order to prevent boundary layer turbulence, it has been found that length L or convergence angle (a) should be set at a Value such that the maximum Reynolds number in the nozzle is less than one million. Alternatively, the nozzle may be cooled to keep the Reynolds number below one million.

In computing the correct length L or divergence angle (b) for section 14 of nozzle 10, care should be taken to avoid making the divergence angle ([1) so large or length L so small that separation occurs between the high-speed gas column and the interior walls of section 14. If this separation occurs, the gas will no longer be contained by the walls of section 14 and the nozzle will no longer control the speed and pressure of the gas.

Accordingly, in computing the divergence angle (b) or length L, a compromise should be reached between the need to maximize this angle (b) or minimize this length L to avoid turbulence in the boundary layers, and the need to minimize angle (b) or maximize length L to avoid separation. The determination of the angle (1)) or length L at which separation will occur may be made by procedures well known in the art of high-speed-gas dynamics. In nozzles of a size practical for use in sonic generators, a divergence angle (b) of approximately 15 has proved successful.

Any gas may be used to operate generators embodying the present invention, and such generators may be operated in any ambient gaseous medium. However, air is most commonly used as an operating gas, and as a medium.

It is believed that when the nozzle 10 is constructed and operated in accordance with the foregoing description, a generally converging, conically-shaped, oblique, compressional shock wave is set up in the gas flowing through the nozzle. Reference numerals 28 generally indicate the probable outlines of such a shock wave; these outlines intersecting at a point 30. A generally diverging, conically-shaped, reflected shock wave 32 is believed to start at point 30 and, but for the presence of pulsator cavity 20, a train of alternately converging and diverging comically-shaped shock waves would continue and, eventually, return the pressure of the over-expanded gas in the jet to ambient pressure. The sonic pressure waves generated by directing the jet into cavity 20 are believed to be caused by a rapidly alternating build-up and release of gas pressure in the cavity. In the embodiment of the invention shown, these waves emanate from the open sides of pulsator unit 18 (see FIGURE 1).

In accordance with the present invention, pulsator unit 18 is constructed and mounted with respect to nozzle 10 so as to permit maintenance of the power output and efiiciency of the sonic generator at optimum values. Specifically, pulsator unit 18 is attached to the body of nozzle 10 by means of two relatively slender, oppositely positioned, leg-like supporting members 17 connected to an annular section 19. This annular section 19 surrounds and is fastened to the body of nozzle 10 by means of screw threads 34. The slender configuration and positioning of supporting members 17 helps provide a balanced, relatively unobstructed sonic pressure wave output from the generator while the provision of screw threads 34 per mits adjustment of the distance Y of cavity 20 from exit opening 26 of the nozzle. Adjustment of the distance Y in accordance with the present invention provides precise positioning of the rear wall 36 of cavity 20 with respect to nozzle 10, and has been found to maximize the generators output power and efiiciency.

It has been determined that the optimum value for Y, the distance between the entrance of pulsator 20 and the exit end of nozzle 10, is given by the following empirical equation:

D*=The diameter of the nozzle conduit at the position where the Mach number of the gas in the nozzle:1.0;

A= /A* the ratio'of the cross-sectional area of the exit opening of the nozzle to the corresponding area of the nozzle conduit having a diameter D*;

(b)=the angle of divergence of section 14 of nozzle 10.

In order further to maximize the generator power output, the depth 2 of pulsator cavity 20 should have a specific relationship to the distance between adjacent points of intersection of the above-described shock-Wave structure, which distance is known as the wave-length of this structure. The relationship between the Wavelength A and the characteristics of the flow through the nozzle is given by the following empirical equation:

D =the internal diameter of nozzle 10 at the position where the gas pressure is equal to the ambient pressure of the gas surrounding the nozzle.

M =the Mach number of the gas flowing at the nozzle exit opening 26.

In this relationship, D may be determined from Equations 1 and 2 above.

It has been found that the generators power output is maximized when the pulsator depth Z is made equal to to approximately three-eights of the wave-length of the shock wave structure. When pulsator depth'Z thus is made equal to the rear wall 36 of pulsator 20 may then be located (with the aid of Equation 3 precisely with respect to the exit end of nozzle 10.

As is well known, the frequency of the sonic wave output of the generator can be varied by varying the depth Z of pulsator cavity 20.

The power output and efficiency of the sonic generator can be even further maximized when the diameter D of the pulsator cavity 20 bears a specific relationship to the diameter D of the nozzle exit, namely that diameter D of the pulsator cavity preferably should not exceed 75% of the diameter D of the nozzle exit, i.e. D .75D

The operating characteristics of four examples of nozzles built and operated in accordance with the present invention are shown in the table below. The input air pressure and the output power of each of the units tested vary from relatively low values (4 p.s.i.g. and 252 watts) for the unit shown in Example 1, to relatively high values (30 p.s.i.g. and 794 watts) for the unit shown in Example 4.

'6 second. The sonic power output was measured at the source.

Although specific preferred embodiments of the invention have been set forth in detail, it is desired to emphasize that these are not intended to be exhaustive or necessarily limitative; on the contrary, the showing herein is for the purpose of illustrating the invention and thus to enable others skilled in the art to adapt the invention in such ways as meet the requirements of particular applications, it being understood that various modifications may be made without departing from the scope of the invention.

I claim:

1. A gas-operated pressure wave generator, said generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway, first and second longitudinal positions in said body member, restrictor means reducing the cross-sectional area of said gas flow passageway and forming a reduced orifice at said first longitudinal position, expansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area for said passageway between said orifice and said second longitudinal position in the direction of flow of gas through said nozzle, resonator means, and means for positioning said resonator means adjacent the exit opening of said gas passageway, the ratio of the cross-sectional area of said gas passageway at said second longitudinal position to the cross-sectional area of said gas passageway at said first longitudinal position being at least 1.5.

2. Apparatus as in claim 1, in which the length of said expansion means between said orifice and said second position is of a magnitude preventing the Reynolds number of the gas flowing near the interior surfaces of said expansion means from exceeding one million while simultaneously preventing any substantial separation of said flowing gas from said interior surfaces.

Example 1 Example 2 Example 3 Example 4 4 p.s.i.g 12.87

20 p.s.i.g 17.55

Frequency of sonic output.

Approx. sonic power output.

6000. 6000 400 watts 6000 500 Watts 30 psig. 30. 6. 59 F. 30.

0.304 in. 0.336 in. 0. 0. 0. 2

o: 23.35 p.s.1.a.

6000 c.p.s. 800 watts.

245 in. 300 in.

79 p.s.i.a. 74

Where The frequency of the sonic output is given in cycles per 3. A gas-operated pressure wave generator, said generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway, first and second longitudinal positions in said body member, restrictor means reducing the cross-sectional area of said gas flow passageway and forming a reduced orifice at said first longitudinal position, expansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area for said pas;- sageway between said orifice and said second longitudinal position in the direction of flow of gas through said nozzle, resonator means, and means for positioning said resonator means adjacent the exit opening of said gas passageway, the rate of increase in area from said orifice to said second position being less than the rate which would cause separation of the gas flow from the walls of said expansion means between said orifice and said second position, the distance between said orifice and said second position being suflicient to produce acceleration of a gas to supersonic velocities, resonator means, and means for positioning said resonator means adjacent the exit opening of said nozzle.

4. Apparatus as in claim 1 in which the space within the walls of said passageway in any cross-sectional plane through said expansion means is substantially open.

5. A gas-operated pressure wave generator, said generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway, first and second longitudinal positions in said body member, restrictor means reducing the cross-sectional area of said gas flow passageway and forming a reduced orifice at said first longitudinal position, expansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area for said passageway between said orifice and said second longitudinal position in the direction of flow of gas through said nozzle, resonator means, and means for positioning said resonator means adjacent the exit opening of said gas passageway, said passageway having the shape of a frustrum of a cone between said orifice and said second longitudinal position.

6. A gas-operated pressure wave generator comprising a nozzle body having a gas passageway therethrough including a frustro-conioal converging inlet section, a frustro-conical diverging outlet section, and a throat connecting said converging and diverging sections, the ratio of the cross-sectional area of said gas passageway at the exit of said nozzle being at least 1.5 times as great as the cross-sectional area of said gas passageway in said throat, a resonator member forming a cylindrical resonator cavity, and support means secured to said nozzle body and said resonator member to position said resonator cavity with its open end facing said outlet section of said nozzle.

7. The method of continuously generating pressure waves in a gaseous ambient medium, said method comprising the steps of continuously expanding and accelerating a gas to form a supersonic gas stream flowing in said ambient medium, the supersonic flow in said stream producing shock waves in a region of said stream, controlling said expansion and acceleration step to create an inrush of ambient gas into a region of said stream, and performing resonant amplification of said shock waves produced in said stream.

8. A method of controlling a gas-operated continuousoutput sonic generator having nozzle means for continuously converting a compressed gas into a supersonicspeed gas jet flowing in a gaseous ambient medium, and plusator means intercepting said jet, said method comprising maintaining the absolute pressure P of said compressed gas within an range in which P, the absolute pressure of said gas jet at the exit of said nozzle means, is always less than the absolute pressure of said gaseous ambient medium, the upper limit of said rangecorrespending to the value for P0 given by the following equation when the value for P is adjacent but below the value of said pressure of said gaseous ambient medium:

In which:

M is the Mach number of said gas jet at said nozzle exit; and k is the ratio of specific beats for said gas in said jet.

9. A gas-operated pressure wave generator, said generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway, first and second longitudinal positions in said body member, restrictor means reducing the crosssectional area of said gas flo-w passageway and forming a reduced orifice at said first longitudinal position, ex-

pansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area for said passageway between said orifice and said second longitudinal position in the direction of flow of gas through said nozzle, resonator means, and means for positioning said resonator means adjacent the exit opening of said gas passageway, said generator being adapted to be supplied with a compressed gas, to convert said compressed gas into a high-speed gas stream flowing in an ambient gaseous medium, and to generate pressure waves in said medium when the pressure of said compressed gas is less than the approximate value of P0 given by the following equation when P equals the absolute pressure of said gaseous ambient medium and M is equal to 1.0:

In which: P0 is the absolute pressure of said compressed gas, and k is the ratio of specific heats for said compressed gas.

10. Apparatus as in claim 9 in which said compressed gas is air.

11. A gas-operated pressure wave generator, and generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway, first and second longitudinal positions in said body member, restrictor means reducing the cross-sectional area of said gas flow passageway and forming a reduced orifice at said first longitudinal position, expansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area for said passageway between said orifice and said second longitudinal position in the direction of flow of gas through said nozzle, resonator means, and means for positioning said resonator means adjacent the exit opening of said gas passageway, said nozzle being adapted to produce and issue into a gaseous ambient medium a high-speed gas jet having oblique shock waves forming a periodic shock wave outline pattern, the first of said shock waves downstream from said exit opening of said gas passageway being a compressional shock wave, said resonator means having a reflecting surface located within said first shock wave.

12. Apparatus as in claim 11 in which said gas flow passageway has a substantially circular cross sectional shape, and in which said reflecting surface is located between A and A downstream from the plane of location of a diameter D which is the diameter of said gas passageway at the position where the pressure of the gas in said passageway equals that of said gaseous ambient medium, where A is given approximately by the following equation:

In which M is the Mach number of the gas in said jet at said exit of said passageway.

13. Apparatus as in claim 11 in which said reflecting surface is located between the first intersection point and the end of said first wave in said shock wave outline pattern.

14. A gas-operated pressure wave generator, said generator comprising, in combination, a gas-accelerating nozzle comprising a body member forming a gas flow passageway having a circular cross-sectional shape, first and second longitudinal positions in said body member, restrictor means reducing the cross-sectional area of said gas flow passageway and forming a reduced orifice at said first longitudinal position, expansion means in said gas flow passageway between said orifice and said second longitudinal position, said expansion means providing an increasing cross-sectional area tfor said passageway between said orifice and said second longitudinal position 9 in the direction of flow of gas through said nozzle, said nozzle being adapted to convert air compressed to pressures lower than 1.9 times atmospheric pressure into a high-speed air jet flowing in the atmosphere, the pressure in the jet at the exit of said nozzle being below atmospheric pressure, a pulsator member with a pulsator cavity in it, and means for positioning said pulsator member with said cavity intercepting said jet and with the rear wall of said cavity being located downstream from the exit end of said nozzle at a point between 7\ and 7t distant from the plane of location of the nozzle diameter D where 7t is given approximately by the following equation:

In which: 1 D equals the internal diameter of said nozzle at the position where the pressure of the air in said nozzle equals atmospheric pressure; and M equals the Mach number of the air in said jet at said nozzle exit.

References Cited by the Examiner UNITED STATES PATENTS 10 Bliss 116137 X Negro 239-601 Daggett 116112 X Amy 116137 Huss 158-73 Stoesling 116137 X Wellenstein 116137 Harper 116137 X Yellott et a1. 116137 Pesce 23973 Straughn et all 117-100 Fortman 116137 FOREIGN PATENTS LOUIS I. CAPOZI, Primary Examiner. 

1. A GAS-OPERATED PRESSURE WAVE GENERATOR, SAID GENERATOR COMPRISING, IN COMBINATION, A GAS-ACCELERATING NOZZLE COMPRISING A BODY MEMBER FORMING A GAS FLOW PASSAGEWAY, FIRST AND SECOND LONGITUDINAL POSITIONS IN SAID BODY MEMBER, RESTRICTOR MEANS REDUCING THE CROSS-SECTIONAL AREA OF SAID GAS FLOW PASSAGEWAY AND FORMING A REDUCED ORIFICE AT SAID FIRST LONGITUDINAL POSITION, EXPANSION MEANS IN SAID GAS FLOW PASSAGEWAY BETWEEN SAID ORIFICE AND SAID SECOND LONGITUDINAL POSITION, SAID EXPANSION MEANS PROVIDING AN INCREASING CROSS-SECTIONAL AREA FOR SAID PASSAGEWAY BETWEEN SAID ORIFICE AND SAID SECOND LONGITUDINAL POSITION IN THE DIRECTION OF FLOW OF GAS THROUGH SAID NOZZLE, RESONATOR MEANS, AND MEANS FOR POSITIONING SAID RESONATOR MEANS ADJACENT THE EXIT OPENING OF SAID GAS PASSAGEWAY, THE RATIO OF THE CROSS-SECTIONAL AREA OF SAID GAS PASSAGEWAY AT SAID SECOND LONGITUDINAL POSITION TO THE CROSS-SECTIONAL AREA OF SAID GAS PASSAGEWAY AT SAID FIRST LONGITUDINAL POSITION BEING AT LEAST 1.5. 